The vertebrate neural crest provides a useful model system for analyzing the problems of cell determination, the regulation of cellular phenotypic expression, and the cellular control of morphogenetic movements during early embryonic development. We propose to study these problems by culturing embryonic chick and mouse neural crest, and sensory and sympathetic ganglion cells in vitro, and analyzing the various environmental factors that promote or permit the differentiation of specific neural crest phenotypes such as melanocytes, sensory or sympathetic neurons. These factors include cell dispersion or interaction, presence or absence of NGF, and other nutritional or topographical conditions. In addition we propose to apply to this analysis several putative neural crest mutants in the mouse embryo, to examine the questions of when and under what conditions specific neural crest genes begin to function, and what the earliest genetic lesion is in the mutant-bearing embryos.